Infection by enveloped viruses is initiated by binding of the viral envelope glycoprotein(s) to specfic receptor molecules on the target cell, followed by fusion between the viral and cellular membranes. We have been studying various aspects of viral envelope glycoprotein/ receptor interactions: 1) HIV env glycoprotein/CD4 interactions. We have developed a novel vaccinia-based assay in which fusion between env-expressing and CD4-expressing cells leads to reporter gene activation by phage T7 RNA polymerase in the cytoplasm of the fused cells. We previously demonstrated that HIV-1 fusion is dependent on the presence of a human-specific accessory component of the CD4- expressing target cell. Our recent findings have refuted the proposal by others that the cofactor is the CD26 antigen. We have initiated approaches to identify the human-specific accessory component. Similar approaches are being applied to characterize cellular components responsible for the cell type-specific fusion specificities which we previously demonstrated for envs from T cell line-tropic vs. macrophage-tropic HIV-1 isolates. We have also examined cellular requirements for proteolytic processing of the env precursor. We found that functional env is produced in the LoVo cell line, which is known to be resistant to protein toxins due to mutation in the gene encoding furin. These results argue against the proposal of others that this protease is essential for env processing. We have continued studies of CD4-PE40, a genetically engineered hybrid toxin that we designed to selectively kill HIV-infected cells. Phase I clinical trials with CD4-PE40 are near completion; unfortunately there is no evidence of therapeutic benefit in HIV-infected adults. Other applications of this agent are still under consideration, including treatment of different categories of infected individuals (e.g., newborns of infected mothers) and ex vivo protocols to eliminate HIV-infected cells in the context of gene therapy and bone marrow transplantation approaches to combat AIDS. 2) Paramyxovirus fusion (NEW INITIATIVE). We have initiated mechanistic studies of fusion mediated by glycoproteins of paramyxoviruses: SV5, measles virus (MV), and canine distemper virus (CDV). In all cases, cell fusion requires co-expression of the virus fusion (F) and hemagglutinin (HA or HN) glycoproteins on the surface of one cell, as well as the presence of the appropriate receptors on the other cell. For the morbillaviruses MV and CDV, fusion occurs efficiently with heterologous expression of F from one virus and H from the other; cell-type specificity is determined by H. We also obtained direct evidence for functional and structural interaction between measles H and CD46, the known MV receptor.